In disk memory systems for use in data processing apparatus, it is usually desirable to provide a write clock signal from an oscillator which is synchronized with the rotational speed of the data storage disk used therein. In some prior art systems in which the clock signal is not so synchronized, variations in the disk rotational speed require that sufficient space be available on the sector regions of a disk to accommodate all of the data which is to be stored therein when the disk speed is at a maximum. Consequently when the rotational speed is below its maximum, a portion of each sector region is left unused and is wasted. Such speed variations may arise because of changes in either the amplitude or the frequency of the line voltage which is utilized as the power source for the disk drive motor.
In order to make use of the entire region of each sector of a data disk and, thus, avoid wasted disk storage area, other prior art systems have been designed to provide for a precisely controlled disk spindle speed. Such designs, for example, may use synchronous motors or non-synchronous motors having relatively elaborate electro-mechanical servo systems to bring about the required precision control. Such solutions are often too costly for most data processing systems.
Accordingly, in order to avoid the presence of wasted storage area or of costly spindle speed control systems, other prior art systems have provided means for generating write clock signals which are synchronized to the rotational speed of the disk so that variations in the rotational speed will be, in effect, tracked by the clock signal. Such techniques normally require the use of pre-recorded information on a portion of the disk storage pack. This information may often be in the form of available data by which the radial position of a read/write head can be determined. In some cases such data is already present for use in the servo system which provides for correct radial head positioning. It has been found that such data can often be appropriately used as an input to a synchronizing signal which thereupon produces the desired write clock signal.
Other techniques for using pre-recorded information involve the use of a pre-recorded clock track on a separate disk which is dedicated solely to the provision of timing information. Such clock track requires an additional read channel and an additional clock read head which adds to the overall cost and complexity of the disk storage system.
In order to avoid the need for pre-recorded data still other systems have used phase-locked loop techniques, as disclosed, for example, in U.S. Pat. No. 3,898,690 issued to A. K. Desai on Aug. 5, 1975. As can be seen therein, an external rotational transducer in the form of a uniformly slotted disk member mounted on the memory disk spindle shaft and a sensor element mounted adjacent thereto is used to provide basic rotational speed and position information. The transducer produces a pulse signal, the frequency (or pulse repetition rate) of which is equal to a fixed, integral multiple of the number of sectors on the magnetic data disks which are mounted on the same spindle shaft. The pulse output signal is supplied to a phase-locked loop which includes a voltage controlled oscillator for producing a pulse signal which is, in turn, an integral multiple of the pulse output signal from the transducer. The phase-locked loop assures that the frequency and phase of the VCO output signal is exactly synchronized with the pulse output signal from the transducer and, hence, is in turn synchronized with the rotational speed of the data disk. In addition, an extra slot is provided on the rotational transducer disk member to provide an index pulse signal for determining a reference peripheral position on the data disk. The system also provides sector timing signals for identifying the sector regions of the data disks.
The Desai system, however, has certain disadvantages which make it undesirable for use in some applications. The phase-locked loop requires an accurate determination of the phase error, i.e., the phase difference between the input signal and the loop feedback signal and then must utilize a difference amplifier and elaborate high-order filtering to assure that the loop responds only to low frequency variations of the disk file rotational speed, to avoid responding to high frequency variation of slot-to-slot time jitter. Such additional components make the system more costly than is desired in many applications.
Further, the Desai system, by using an extra slot for indexing, subjects the system to indexing errors which can arise when an extraneous transient pulse, not synchronized to the uniform slot pulses, occurs at the sensor output. Such a transient pulse is incorrectly interpreted as an index pulse by the Desai system and causes an incorrect indentification of the data disk sectors.
Moreover, while the Desai system provides a system for sectoring the rotating memory, there is no relationship shown among any of the signals involved in the sectoring process and the "write clock" signal used for clocking data onto the disk memory. Such write clock signal must, therefore, be provided by some other means and suitable synchronizing logic employed for providing the desired timing relationship among the write clock signal and the index and sector signals of Desai's sectoring loop configuration.
It is desirable that the overall sectoring and indexing operation be performed in a less costly manner while, at the same time, avoiding any response to "false" indexing information due to random transient pulses which may arise during operation. Further, it is desirable to provide for a write clock signal directly from the phase-locked loop operation so that further time synchronization logic for synchronizing an independently generated write clock signal with the sectoring and indexing signals is not required.